Vasoconstriction, Vasodilation, and Blood Physics - HELP!

[waffle23]

Ok so starting off with blood physics, I know from fluid mechanics that ideally, when area is increased, velocity decreases from Q=Av; and this causes pressure to increase from Bernoulli's equation, so area and pressure are each inversely proportional to velocity and area and pressure are directly proportional to each other.

However, why is it that vasoconstriction increases blood pressure? If the blood vessels are constricted, area is reduced, so shouldn't pressure decrease too? I know that this is not the case, but how come it is the reverse of what is expected from ideal flow based on the principles of fluid mechanics? Also, even though pressure actually increases in this case, does velocity still increase from what is expected in the continuity equation?

Also, can someone help me on understanding the respective functions of vasocontriction and vasodilation? I know that these can be used to shunt blood flow. For example, would vasodilation in the muscles during exercise help in increasing activity and oxygen transport due to overall increased blood flow, or would it decrease activity because of decreased velocity?

Also for vasoconstriction and vasodilation, what happens with heat? Does vasodilation of the skin blood vessels have a cooling or warming effect? I think it is cooling since you get red when you are hot, but why exactly does dilation have this effect, and why does constriction cause you to warm up exactly? I guess in this way - how exactly does the circulatory system function to control body heat? When the skin vessels constrict or dilate to control heat, are other vessels in the body antagonistically constricted or dilated because of this?

Is there anything else about blood flow physics, vasoconstriction, or vasodilation that would be helpful to know, particularly specific function like the heat control and different types of activity? Thank you!

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[rocuronium]

This topic has been popular recently, and some very well written posts have been made which explain in detail the questions you are asking.

I suggest that you perform a thorough search of this forum. It is likely that you will find the answers to all of your questions and more.

 

[Charles_Carmichael]

As gleek mentioned, please take a look in the Q&A subsection. There have been several topics regarding this recently.

I guess I'll explain anyways. You can't apply the principles of ideal fluids to a non-ideal fluid such as blood. Vasoconstriction increases pressure upstream and decreases pressure downstream. Think of the following equation:

dP = Q x R

where dP is the change in pressure, Q is the blood flow rate (cardiac output) and R is the resistance. Vasoconstriction increases resistance and at resing conditions, the cardiac output is relatively constant at around 5 L/min. So, so balance out both sides of the equation, with increasing resistance, the change in pressure along the length of the constricted area also has to increase. The increased resistance dissipates more energy (and pressure can be thought of as potential energy) so the downstream pressure is lower than normal.

Regarding your second question, vasodilation of blood vessels in skeletal muscle and vasoconstricton of blood vessels leading to visceral organs shunts more blood to the muscles during physical activity. Vasodilation/vasoconstriction don't really control the blood flow. The equation for cardiac output is: CO = HR x SV where HR is the heart rate and SV is the stroke volume (amount of blood pumped per heart beat). During exercise, both of these factors increase to increase the blood flow rate. Vasodilation/vasoconstriction regulate the resistance that the blood flow faces and during exercise, you want as little resistance in the blood vessels of skeletal muscles as possible to maximize oxygen delivery and CO2 clearance.

With heat, vasodilation of the blood vessels in the skin "exposes" the warm blood to the environment. If the surrounding temperature is cooler (and it likely will be in most conditions), heat will escape from the blood vessels into the environment (it travels from the area of higher temperature to the area of lower temperature). If the blood vessels in the skin are constricted, very little blood flows through them and is exposed to the colder surrounding temperature. This minimizes the amount of heat lost from the blood and, thus, the body.

Remember, at any point in time, not all capillary beds are perfused; rather, they are selectively perfused based on metabolic needs. So dilation of the blood vessels in the skin could mean that other vessels might be constricted. However, I don't have an absolute answer for that question. Hope this helps.

 

[plzNOCarribbean]

Soooo confused about this.

So, basically, the reason we cant compare fluid flowing through a pipe to fluid flow in a biological system such as the cardiovascular system is because in the biological system fluid flow is not idea?

With fluid dynamics, its safe to assume and follow the golden rule that pressure is greatest in the area with largest cross sectional area (even though FLOW rate is constant throughout the pipe). Basically, P is proportional to the cross sectional area, A and inversely proportional to v?

^This is correct right? Despite the equation P= F/A? CAN someone PLEASE answer this!

but in terms of the cardiovascular system, we say that BP increases even though A is decreasing, which is the complete opposite of what I just said regarding fluid from a physics perspective. And so, your saying that this can be attributed to the resistance in the vessels due to the decreased diameter during vasoconstriction? So we disregard relating all of the physics formulas to this concept because this can explained by the equation dP= Q x R, and as R increases due to decreased diameter, pressure must also increase??????

PLEASE HELP. I've been trying to get this straightened out for days. I'd really appreciate the help especially since I'm about to start doing a ton of fluids and cardiovascular practice problems

 

[Adlogin]

Arteries, veins etc. don't behave like pipes. The heart generates the pressure throughout the cardiovascular system and the vessels increase their diameter when subjected to high pressure.

 

[plzNOCarribbean]

Arteries, veins etc. don't behave like pipes. The heart generates the pressure throughout the cardiovascular system and the vessels increase their diameter when subjected to high pressure.

Yeah but I was specifically referring to the result of reducing diameter. What your saying is that an increase in pressure, for whatever reason (increased reabsorption of water, increased peripheral resistance, increased stroke volume) will be compensated by vasodilation?

 

[Whiteshoes]

Yeah but I was specifically referring to the result of reducing diameter. What your saying is that an increase in pressure, for whatever reason (increased reabsorption of water, increased peripheral resistance, increased stroke volume) will be compensated by vasodilation?

Yea i'm confused about this too.. so i guess the bottom line is when considering blood vessel pipes the Bernoulli equation doesn't apply because blood is not an ideal fluid. idk.. this post i quoted below sort of clarified it for me i think.

Okay, I figured it out...I didn't put it in terms of resistance, but I point out when pressure increases or decreases. the Q=AV doesn't give the pressure relationship, Bernoulli's equation does. basically, the resistance concept doesn't apply when looking at one specific site of a vessel. when the resistance does apply, Bernoulli's equation does not work because that equation is based on an ideal fluid, which has 0 resistance. just read below.

Based on Bernoulli's equation and volumetricflowrate=Av, velocity increases, cross-sectional area decreases, and pressure decreases. This can be applied only in one specific region of a vessel. In a question that compares a health artery to a nonhealthy artery with a blockage due to atherosclerosis, you use the Bernoulli relationship between pressure, velocity, and CX-area when looking at the exact region of the 2 arteries. So, the blockage decreases CX-area, velocity goes up, and blood pressure decreases AT the site of the blockage; however, systemic blood pressure increases because the heart is working to pump out more blood volume in order to maintain the volumetric flow rate.

When it comes to comparing different areas of a vessels to one another, you cannot use Bernoulli's equation. In this case, pressure and CS-area do not both increase or decrease. Compare an artery to the capillaries. Based on Bernoulli's equation, the capillaries have a larger surface area, thus they should have higher blood pressure. We know this is not true. Capillaries have the least amount of blood pressure of any blood vessel. Therefore, we CANNOT apply Bernoulli's equation when comparing different areas of vessels. This is because Bernoullli's equation is based on an ideal fluid flow, and blood is not an ideal fluid. HOWEVER, the constant volumetric flow rate relationship with CX-area and velocity does still apply. Because volumetric flow rate is constant, capillaries have larger surface area thus should have a slower velocity, which is true. Blood moves the slowest in the capillaries compared to any other vessel.

When it comes to vasoconstriciton, CX-area decreases and blood pressure increases. This also means Bernoulli's equation cannot be applied with vasoconstriction and vasodilation. You can only apply it in blood vessels when you are looking at one specific region of a vessel. Constantvolumetricflowrate=Av always applies, to my knowledge.

Now, if the passage tells you to assume blood is an ideal fluid, then you do apply Bernoulli's equation.

Does this help?

 

[Bumbl3b33]

Yea i'm confused about this too.. so i guess the bottom line is when considering blood vessel pipes the Bernoulli equation doesn't apply because blood is not an ideal fluid. idk.. this post i quoted below sort of clarified it for me i think.

I'm a little unclear as to what the question is but if I may interject:

Yes, blood flow is NON-ideal. There is a lot of turbulence, and moreover the flow is made up from a non-ideal fluid. Yes, bottom line is, Bernoulli doesn't apply here. In a plain scenario, meaning nothing is being done to alter anything outside of normal heart function: Pressure in the cardiac system is highest in arteries (it's getting a large volume of blood shoved into it from the heart) followed by capillaries, followed by veins (so, pressure A>C>V). Area wise, capillaries by far have the greatest amount of CROSS-SECTIONAL area meaning that every capillary is not big (in fact they're quite small) but when you have billions of capillaries, they amount for a great chunk of area, followed by arteries, followed by veins (so, area C>A>V). Now, velocity wise, as I mentioned earlier, arteries get it the fastest straight from the heart, followed by veins which has considerably slower moving fluids (that's why blood can pool in your leg if you don't move around), followed by capillaries, which makes sense because capillaries are about 1-cell in diameter so cells diffuse into the interstitial fluid 'one at a time' quite literally you can think of it that way if you wish. If pressure increases in an artery, vessels will probably dilate to compensate for the pressure. Remember that MCAT is looking for basics; in reality, keep in mind that arteries are not very distensible (they're stiff) because they have more layers of smooth muscle around them compared to veins which change a lot more if they were subjected to the same change in pressure, AND if the blood pressure is too high the heart can also stop pumping as hard/decrease its stroke volume (amount of fluid it pumps out per contraction).

Like I said, MCAT is looking for basics, so keep it simple. "Bottom line" is, say you're given one of those wretched problems that's like. Oh hey, I just pinched the crap out of someone's artery, what is going to happen?!?! 😀 Well, so think of one of those balloons that clowns use to make balloon animals. If you squeeze one part of it, what happens? everywhere that there is no force being put, expands, or in our smarty-pants terminology, pressure drops! Well, if pressure drops, and flow rate is kept constant (which it is in this case, right? The heart hasn't changed how hard it's pumping) we know from our balloon that the area increase, ergo, the velocity decreases! Yaaaayyy! Keep it simple fellas, and I promise you can't go wrong.

I know I presented two different frameworks for the sake of clarity, but for any manipulations, always think like a clown!

BTW, There is a great diagram in EK Bio Lecture book that puts all the area, pressure, velocity correlations into a graphical format, if you can get your hands on it.

Best of luck, and if I was wrong on any point or you need me to clarify, let me know! <3

 

[Flamen04]

Another minor thing is that blood capillaries are often arranged in parallel. Therefore, a capillary bed can dilate/constrict without affecting the blood flow of other regions of the body.